模電資料庫---單片機的各類例程和指導--11_WhyOpAmpsOscillate--anintuitivelookattwofrequent

1、Why Op Amps Oscillate an in tuitive look at two freque nt causesBruce TrumpBode plots are great analytical tools but you may not find them intuitive. This is a purely intuitive look at frequently encoun tered causes for op amp in stability and oscillati ons.The perfectly damped response in figure 1

2、occurs with no delay in the feedback sig nal reachi ng the inv ert ing in put. The op amp responds by ramping toward the final value, gently slowing down as the feedback signal detects closure on the proper output voltage.Problems develop whe n the feedback sig nal is delayed. With delay in the loop

3、, the amplifier does not immediately detect its progress toward the final value. It overreacts by racing too quickly toward the proper output voltage. Note the faster in itial ramp rate with delayed feedback. The inverting in put fails to receive timely feedback that it reached and passed the proper

4、 output voltage. It overshoots its mark and requires several successively smaller polarity correcti ons before fin ally settli ng.A littledelay and you merelyget some overshoot andringing.Too much delay and these polarity correcti onscon ti nue in defi nitely an oscillator.The source of delay is oft

5、e n a simple low-pass R-C n etwork.Okay, it not a constant delay for all frequencies but the gradual phase shift of this n etwork from 0produces to first-order approximation of time delay, td=RC.There are two com mon ly encoun teredsituatio ns where thisR-C n etwork uninten ti on ally sn eaks into o

6、ur circuits. The firstis with a capacitive load (figure 2a). The resistor is the op amp s ope n -loop output resista nee. The capacitor is, of course, the load capacita nee.In the second case (figure 2b), the feedback resistance and the op amp s in put capacita nce form the R-C n etwork. Circuitboar

7、d conn ecti ons alsocon tribute to the capacita nce at this sen sitive circuit no de. Note that the two circuits have ide ntical feedback loops. The only differe nee is the node at which the output is taken. From a loop stability standpoint, they can create the same issues. And these two causes of d

8、elayed feedback ofte n occur in comb in ati on a bit of both can be double trouble.The second case needs a bit more comment:A feedbackresistor is not n eeded for the simple G=1 buffer so the more com mon situati on is in a gain con figurati on using a feedback resistor and resistor to ground (figure 3). The parallel comb in ati on of these resistors forms the effective R in the R/C circuit.There s more to be learned with Bode analysis of feedback amplifiers. Still, this simple intuitive view of how delay or phase shift in the feedback path affects stability can